Fossil bone or ivory, also known as Bone Turquoise or Fossil Turquoise, generally obtained from mastodon, mammoth, dinotherium or other extinct animals, which are naturally deep blue coloured by impregnation of vivianite, an iron phosphate. The chemical composition is near to apatite.

It resembles turquoise and is used as imitation for turquoise. Sometimes, but rarely, it is coloured green by copper. It is cut and polished for jewelry but loses its colour in the course of time.

Also known as odontolite, this beautiful rock is formed in the depths of sedimentary stacks, as bones like this fossil jaw are preserved for the aeons during the process that turns their surrounding sediments into rock. Teeth are the most common part of the skeleton that turn into odontolite, hence its name. It was long thought to be coloured (like turquoise) by copper, and when analysis failed to reveal its presence, by the iron phosphate mineral vivianite (seehttp://tinyurl.com/mnvy6vx). Recent research has shown that it consists of crystals of blue apatite (seehttp://tinyurl.com/l5rzpag), coloured by trace impurities of manganese.

In the old days it was called occidental turquoise to distinguish it from the oriental version which is a clay rock coloured by copper oxide, and occasionally the former has been erroneously (or fraudulently) sold as the latter. They can be told apart by their hardness, with the oriental version being harder. The most famous locality is in France, especially in the southwestern Languedoc region, where they accumulated in river sediments derived from the erosion of the nearby Pyrenean chain. They preserve bone structure beautifully and are much sought after by palaeontologists.

The jaw in the photo (size 5.7 x 5.3 x 3.3 cm) is recent, and belonged to a Pleistocene rodent…